Film-screen x-ray imaging devices employing photographic film are widely used for medical imaging. However, the film is often overexposed in some areas and underexposed in other areas clue to the limited range of contrast of the film combined with the thickness and composition variations of the tissue across the image. Discrimination of contrast differences of soft tissue in the overexposed and underexposed areas of the film can be difficult. This problem is especially apparent in film-screen mammography.
Attempts have been made at replacing film with electronic image sensors. Potential advantages of electronic image sensors over film include more accurate measurement of x-ray intensity over greater ranges, ability to digitize the image data, ease of archiving and transmitting image data, and improved display capabilities.
However, widespread clinical deployment of digital x-ray radiology has been hampered by the lack of a relatively inexpensive, compact, digital x-ray image sensor of sufficient image size and resolution. Present digital x-ray imaging systems typically use a fluorescing plate that converts each x-ray photon into a large number of visible light photons to produce a visible light image. The visible light image is then imaged onto an optical image sensor such as a CCD. The imaging performance of these techniques is degraded by relatively low x-ray to visible light conversion efficiencies, low collection efficiencies of the light photons, additional quantum noise from the light photons, and loss of resolution due to light spreading in the x-ray to visible light converter.
It is known that selenium is a photoconductive substance, i.e. x-ray photons absorbed in a layer of selenium exposed to an electric field will create a number of electron/hole pairs permitting a current to flow through the otherwise insulating layer. Xerox Corporation developed an x-ray imaging device in which an x-ray induced charge distribution on a selenium-coated aluminum plate is recorded with a paper/toner process. Philips Corporation presently markets a chest x-ray imager in which an x-ray induced charge distribution on a selenium-coated aluminum plate is recorded with scanning electrometers.
Complimentary metal oxide semiconductor (CMOS) fabrication technology is a well established industry which involves fabricating integrated circuits on and in the upper surface of a wafer of crystalline silicon. CMOS technology utilizes the silicon of the substrate wafer as the semiconductor material for transistor fabrication. The high mobility of charge carries in single-crystal silicon results in fast, compact, low-noise circuitry. Wafers with dimensions as large as six inches are available for large area CMOS circuits.
Thin film transistor (TFT) technology is an emerging semiconductor fabrication technology in which transistors are fabricated using a thin film of semiconductor material such as amorphous silicon, polycrystalline silicon or amorphous cadmium selenide deposited on an insulating substrate. An advantage of TFT technology is the potential for large area circuits. However, the disordered molecular structure of these thin films leads to low charge mobility which limits performance. In comparison with CMOS circuits, TFT circuits are generally slow and noisy with large leakage currents.
Various approaches are presently being proposed and investigated for directly acquiring a digital x-ray image. For example, Zhao and Rowlands (Proc. SPIE 1993; 1896:114-120) have proposed a readout array fabricated using cadium selenide TFT technology with an amorphous selenium coating. Tran et. al. disclose, in U.S. Pat. No. 5,235,195, TFT array circuits coated first with a "planarization" layer which in turn is coated with an energy-sensitive layer.